The production of extracellular inulinase (beta-1.2-D-fructan fructanohydrolase, EC 3.2.1.7) was studied in fed-batch cultures of the yeast Kluyveromyces marxianus CBS 6556 at 30 and at 40 degrees C. At both temperatures? the final biomass concentration exceeded 100 g.l(-1) and more than 2 g enzyme. L(-1) of culture su pernatant was produced. The biomass yield on O-2 at 40 degrees C was substantially lower than at 30 degrees C. Nevertheless, at 40 degrees C a growth rate of 0.20 h(-1) could be maintained for a longer period than at 30 degrees C. The unexpected higher O-2-transfer rate at 40 degrees C is probably due to a lower viscosity of the culture broth. The 40 degrees C fermentation took only 33 h as compared to 42 h at 30 degrees C. These results indicate that K. marxianus is a promising host for the extracellular production of heterologous proteins under the control of the inulinase promoter.